Electromagnetic vibratory device



Sept. 9, 1952 J. s. P. ROBERTON 2,610,259

- ELECTROMAGNETIC VIBRATORY DEVICE Filed Aug. 7, 1947 3 Sheets-Sheet 1 I uentor EM, EM 2 m Myw?" A Horn y S p 1952 J. 5. P. ROBERTON ELECTROMAGNETIC VIBRATORY DEVICE 5 Sheets-Sheet Filed Aug. 7, 1947 I lnve tor w n KM Altar ey Sept. 9, 195 2 J. 5. P. ROBERTON 2,610,259

ELECTROMAGNETIC VIBRATORY DEVICE Fil'ed Aug. 7, 1947 Y 5 Sheets-Sheet 5 llorney Patented Sept. 9, 1952 4 UNITED STATES PA anncmomonmro VIBRAIORY nnvren JamesSamneLPaterson ltobertom l ondon land,.assig nor to International Standard trio, QOIPQratiomNew York, N. Y a corporation of Delaware ApplicationAugusH, 1947; Serial No; 761,241 [InGreatrBritainJune21-1946 Se.c 0nl",, ?uh ic Law 0;, Augusta 1346i Patent expires June 21;,"1966 iwiaim (01.179-114) Thisinventionrelates to electro -acoustic transducers, and more particularly thoueh not chem--- sivelyl to telephone receivers;

Such devices have commonly-=oonsisted of a.

magnetic metal; diaphragm on one side: oi which were placed pole pieces co-operating with a: permanent magnet, such pole pieces: being sur rounded by operating coils.

It iswell knownin the: design of electro -acoustic= transducers of this type that the sensitivity: is' proportional to the product of the polarisingo fiux density in the main air gap between pole pieces:

and the diaphragm; multiplied by the; square root of the effective permeability of the diaphragmby voice frequency fluxes, and therefore tends to increase as the steady magnetic flux: in the I said air gap is increased.

' This-tendency is, however, opposed by the tendenc-y of the diaphragm to become saturated by the steadyfiux so that its permeabilitp is reduced to alternating fluxes corresponding to the sounds. Alimitof sensitivityis thlw set'iorany given com bination of diaphragm-and magnet system;

In: designs of electro aooustic transducers C0m monl-wadopted'in the past-the ratio-of fluxes in the air gap on air gaps between the diaphragm and'the pole pieces :ofthe magnetic system "on the one hand and that in the material of diaphragm;

on the otherhand, has been limited tounity, since both :fieldsfollow the samesorbit; when leakage effects are ignored: 7

It has beeniound that'the'sensitivitywhich can be obtained; for any given limiting desien condt tions, can be increased when the steady magnetic flux thesaid: :air= gapeihaea componenta which does, not pass through the material ofi the ':dia--- phragm' itself or-where ithascomponents passing; through" different parts: oil the material ot' the diaphragm.-

Accordingto=one ofi-ts aspeots the inventionconsists of an electro-acoustie-transducer having a diaphragm, a single permanent magnet member, one or more pole-pieces one i enda 0f such potepi'ece, or-ofeach of them if-more than one; being" adjacent to the diaphragm and iormingone or more .gaps-respectivelybetween-the:- saidri one or more pole-piecesand thediaphragm, snoh one or: more pole-pieces co-operating with the eard Del manentmagnet member to'produce 'a magnetic flux in such 'one 'GlfimOl gaps, and having also an additional poleohpole pieces lac-operating with the said permanentmag-netmemben whereby not*all' ot' theflux 'which -traverses thesaid one or more gaps-traverses that part on those parts of the diaphragm which such flux would traverse inthe absence of the said additional pol'e-piece 0r pole-pieces,

According'to another of' itsaspects, the invention consists of an electro-aooustic transdncer having a diaphragm andasingle permanent msgnet member, in which the magneto-motiye' force:

of the said permanent magnetmember causes a polarising flux in one or more gapsbetween the diaphragm on the one hand andj the said permanent' magnet member-cr che or'n'iormpol'f pieces (ac-operating therewith on the other-hand; andin which a" portion ofthe said magn'etomotive force is used to-divert po1arising -fliix fromt a part or parts of the diaphragm-through which: all the po1arisingflux in the said oneor more gaps would-otherwise pass:

Accordingto another ofits aspects, the inven tion-consistsr of an 'electro-acoustie transducer havinga diaphragm ofmagneticmateriaha-singlm permanent magnet member, one or'more pole pieces, and oneor more additionalpole piecesfthe firstmentioned one or'morepole pieces appmach ing near to the diaphragm soas'torformionezor' more air gaps between the sai qlyone or'more'fii st mentioned pole n c and o epartbn' he ak r gm;,1and t e saidadditio aione" or more ole.

eces peratina with the saidyp rm n m magnet member to provide an additiqnalfmg etic circuit so. that. the ratio bietweehthe tota maenetidflux, in the air gap or air. aps. between the diaphragm and the first mentionedone be more pole pieces-on the-onehand, andtheflngc n that part or those parts of thediaphragai av h, wouldbetrayersedby magnetic were the additional, one or; more Dolepieces to be, remov onthe other hand,- maybeincreasedabove the former "flnxbeing the greater;

Ac ordin to ano her (ants-a pec s thetinv nr ion con ists of an, electrov-acoustic transducer:

' havin ;adi p raemrofmasn tiematerial, asinele permanent: magnet member, 01516101" more; poles piecesw (hereinafter called the -mam 13018;" systemV), and; one or: more additional pote piece (hereinafter calledthe auxiliary "poles systemif) I thesaicl" permanent-magnet member coI-op'eratin with-the main pole system to produceta mastic flux in one or moreair gaps ('here-inafterpcal the main. gap: system). betweenn'the main no system and one part of the diaphragm andithee-g auxiliary epole system (lo-operating with :the

manent magnet member to produce an additionalmagnetic flux in the main gaps.systemsothatonlsii' a part of the flux inth'e main gap system traverses? that part. or parts of the! diaphragm. which would be traversed by the flux in the main gap system in the absence of the auxiliary pole system.

In certain practical embodiments of the invention the additional flux produced by the auxiliary pole system, is applied so that it traverses a gap or gaps, hereinafter called the auxiliary gap system, is pierced by a single hole in the centre for the ingress or egress of sound. It is possible of course for the depressed central portion to present a disc-like face to the diaphragm and for the sound aperture or apertures" to be located elsewhere on the plate.

In Fig. 2 corresponding reference numerals to those used in Fig. 1 are used to denote corresponding things. A round bar magnet I polarised axially, stands in the bottom of a cup 8 of permeable magnetic material. A pole piece 4, similar to pole piece '4 in Fig. 1 stands on top of the magnet.

' A member I of permeable magnetic material,

single part of the diaphragm carries the whole of I Certain embodiments of; the invention will now be described in relation to the attached drawings in which:

.Fig. 1 is a diagram of one embodiment of the invention in cross section Fig. 2 is a diagram of another embodiment of the invention in cross section I Fig. 3 is a diagram of another embodiment of theinvention incross section v V .Fig, 4 is a circuit diagram representing an electrical analogy of the magnetic systems of the embodiments shown in Figs. 1, 2 and 3.

The embodiments in these figures have a pole piece of one polarity, representing the main pole system and such arrangement is hereinafter called a monopolar arrangement.

Fig. 5 is a diagram of an embodiment of the invention having two pole pieces of opposite polarity representing the main pole system and such anarrangement is hereinafter called a bipolar arrangement. Y

Fig. 6 isa circuit diagram representing an electrical analogy of the magnetic system of theembodiment shown in Fig. 5.

'Fig. 7 is a diagram of a second bipolar embodiment of the invention.

Fig. 8 is a circuit diagram representing an electrical analogy of the embodiment shown in Fig. 7.,

Fig.9 is a diagram of a third bipolar embodiment of the invention,

"Fig. 10 is a circuit diagram representing an electrical analogy of the embodiment shown in Fig. 9.

Fig. 11 is a diagram of a fourth bipolar embodiment of the invention.

Figs. 12, 13 and 14 are top view, transverse cross sectionand bottom view, respectively of a telephone earpiece capsule following the arrangement shown diagrammatically in Fig. 1.

-In Fig. 1 a cylindrical magnet l and a diaphragm 2 of magnetic material are arranged so that the diaphragm lies within the magnet a little distance from and in a plane parallel to that of one end face of the magnet. The magnet is polarised axially, one end face being of north polarity and the other end face of south polarity. Across the endface of the magnet furthest from the diaphragm lies an end plate 3 supporting a single cylindrical pole piece t around which are operatingwindings 5. Plate 3 and pole piece t are the main pole system. A plate '6 rests onthe other end face or the magnet and is dished in the centre so as to approach close to the diain the form of a hollow truncated cone with an inward facing flange at its smaller end is fixed by this flange part way down the magnet, nearer to pole piece 4 than to cup 8. The conical member 7 extends in a direction away from the bottom of cup 8 and towards its larger end, which carriesthe diaphragm, so as to leave a gap between the centre of the diaphragm and pole piece 4. A member 8, similar to fi of Fig. 1, may be turned over. along its outer periphery so as to meet the upper rim of cup 8 or alternatively cup 8 could be extended upwardly and plate 6 could be as shown in Fig. 1.

In this arrangement pole piece 4 forms the main pole system as in Fig, 1, but the auxiliary pole system is constituted by cup 8 and plate 6.,

Plate :8 in the Fig. 2 arrangement is capable of the same modifications of central pole face shape and sound aperture as are described in relation to Fig. 1.

In Fig. 3, a flat bar or disc magnet is used polar ised from centre to ends or circumference respectively. V

In the centre of magnet l the pole piece 4 is fixed and a cylindrical member ID, of permeable magnetic material rests, at two places on one of its end faces, on the outer ends of magnet I. A plate 6, similar to plate 6 in Fig. 2, rests on the other end face of cylindrical member ID. Another member 9, of permeable magnetic material which may be a cylinder of a part or parts of marked with the reference A, the auxiliary gap, system with the reference E, the part of the magnet associated with the main pole system, by the reference Vi, the part of the magnet associated with the auxiliary pole system by the reference V2 and the diaphragm by reference D. 1

In Fig. 4 an electrical analogy is represented using these letter references to identify the circuit element symbols.

' VI and V2 are represented by batteries equivj alent to the magneto-motive forces of the respectlve' parts of the magnet, A (the reluctance V ofthe maingap system) by a resistance, E (the reluctance of the auxiliary gap system) by a resistance and D (the reluctance of the diaphragm) by a resistance. 7

V3, a source of alternating current, represents the alternating magneto-motive forces corresponding to the sounds to be transduced.

aeroasa Theapplication Q of network analysis to; Fig 4" would reveal-a somewhat complex pattern of cur-- rentsflowing in the various elements of the circuit but the generaleffect of ithe arrangement 1 is thatthe totaLst-eady current in A is'thesum of the steady currents inE and D, and only the latter passes through thediaphrag-mlaterally- (that is to say in a direction parallel to the diaphragmsurface) so'as to saturate it.

The alternating currents in the system are also partiallydiverted through and V2 which causes some loss but not inproportionto the gain resulting from 1 the increased current in A as compared with the maximum 'feas'ible figure for this in the conventional: type ot electro acoustic transducer.

The matter can be viewed f-romanother standpoint-without recourse to the electrical analogy.

It iswell: known. that the: efficiency of'ran' electro-acoustic transducer of thewtypehaving an armature of magnetic material, is. proportional to the-product of the: steady pol'arising flux density in the gap between. theiarmature and: theLmag-Q. net orpole. pieces co-eoperating. therewith; multiplied.- by. the. square. root ofthe permeability to alternating fluxes, of the magnetic: circuit as. a whole. Where. the. diaphragm: itself. constitutes the; armature, this; magnetic circuit .will. include the.:.diaphr.agzn and an. air 'gap or air: gaps be.

tween theilattersand the. magnet lor pole. pieces.

Increasing the. polarising flux density in the said-gap. or 'gapstbeyond'. a. certain. point; tendsto reduce. the permeability, to alternating...fluxes, ofczthe magneticcircuit, due to saturation' of the.

diaphragm. by thepolarisingflux, andithesmaxi mum efficiencyiifors any .given' design will beattained when the said permeability to alternat-: ing fluxes is somewhat below its maximum value, since. the rise. of-the polarising flux density is more influential; in the. product thanathe saidpermeability since only: the square root of. .the latter isconcerned. in theproduct.

The point at whichit ceases to be; of advantage to increase; the: polarising flux beyond that;

n hB-maxi ummue o rm a t e.

'Ihe vibrational "iorcesacting on "the dia-- phra'gm from the auxiliary gap system" oppose those from the main gap system but not-tothe same: degree as theinc-rease in the latter; and there is-a considerable net gain in 'efiici'ency.

It: is-general1y -advisable, to arrange for the polarising-fi'uxiinthe auxiliary gap-system tobe smaller thanithat in. the main gap" system-toavoid; mechanical instability. 0f1.th6.j diaphragm.

which. might, arise. if a it. "were, pulled; in :opposite.

directions by. two opposing. and nearly equal',

polarising .rnagneto-motiveforces.

Ad nta e ma begt ke r 01f.v e; pr n an mb rltways O urse. w en n e o ust nsdu r i b d ed. at. w th.

15 3 n rma p fi n i 9 0. 9. 1. for all but. the. permanent. magnet the lat-... ter being madelarger. by the. amount necessary q to provide the additional" polarising flux."

Alternatively, if i-t--were-required-for any---rea= son" to use a smaller or thinnerdiaphragm, some of the usual disadvantages. of smaller-*and thinner diaphrag-ms could be avoidedby the use of a normal sized magnet, only part of which -aplied steady fluxto the diaphragm material, the rest of the magnet being-utilised" to produce-"that part of the total fluxes not traversing the diaphragm.

The same principles can be applied-to bipolar types of electro acoustic transducer and Figs; 5 to 1 1 illustrate several such types.

Fig. 5- illustrates diagrammaticallythe-mag: neticcircuitof a bipolar-electroacou-stic'transducerQh-aving am-ag'net" l-l, polarised longitudi nail-y; and with polepieces l2 andi3 attached to. its two ends and extending to the axis-0F themagnet towards a diaphragm I4, so as to" leave a gap between each pole piece; andthe diaphragmr. These gaps form "the main gap system.

Additional pole pieces #5 and I6 are attached to intermediate pointsalong the length of the magnet; equidistant from'theends and |-5'- and? 16" extend away from the magnet beingshaped to pass round the edge of thediaphragmandbent-overso as to approach the diaphragmorr theother side from the polepeices i2 and I3; leaving-gaps between each of the pole pieces ['5' and i6; and the diaphragm which are substantially opposite to the gaps-of themain gap systern. Thegap's between l5 and I6 and the dia.-- phragm form the auxiliary gap system.

The pole pieces lif-and [Kare crossed over so. that I5 isof opposite polarity-tothat of; I 2; and lfiis of opposite-polarity'to that of l3.

Interconnected coils are wound round l2 and I 3-, in which flow-alternating currentsreprescntingsounds. e

The diaphragm is mechanicallysupported by meansnot shown.

Fig. 6 showsan electrical analogy-ofthis ar rangement the similar letter referencesjdenotingf 1 the various circuit elements, to those used in Fig. 4, except that the main gap system and auxiliary gapsystemeach consist of two gaps instead of one, and areiepresenjted as two elements, A /l and two elements" E/ 2'; respectively and the: alternating current source is showna's;

two elements, Vii/2';

These same references are also markedon' 5 to denote elements ofthe magnetic circuit corresponding to the elements of the electrical" analogy. In bipolar arrangements the fluxes; traverse the part of the diaphragm between the points opposite the pole pieces of the main pql'e system.

Analysis of the currentsfl'owingin the yarious;

parts of" the circuit shows that the gaps, A1 2,

of the main gap system each carry current equal to the sum of" the current flowingin' D'plus the; current flowing in one of the gaps'E/z ofthe auxiliary gap system,,that is to say greaterthanj: the current in jD, so that the arrangementis V similar in principle to the monopolar. arrange ment previously described.

Rig. 7 shows an extreme iorm of. the arrange ment' of'Fig. 5 and Fig. 3 shows an electrical analogy thereof; In'these figureslthesamenu Y merical and other. references. are. used as, thQ in Figsfi and 6,. to indicate corresponding ele ments.

The. pole pieces l5 and lfijare here conneetedj acrossj thewhole of magnet, II and inorder th t.

. the. .e.$ ex ti n theldi h a m fr m .thej d l-j rection of the main gapsyste'm shall predominate 7... over those from the'direction of the auxiliary gap system, the desirability of which has been explained, it is necessary to have wider gaps for the auxiliary gap system, than for the main gap system.

Figs. 9 and 10 in which the same numerical references as those used in Figs. and 6, are used to denote corresponding elements, show respectively another bipolar arrangement and an electrical analogy of. the magnetic system thereof.

'Themagnet His magnetised with consequent polarity along its length. The pole pieces l2 and 13 of the main pole system are in contact with intermediate parts of the magnet substantially equidistant from adjacent ends of the magnet and the pole pieces [5 and it of the auxiliary pole system are in contact with the ends of the magnet.

.The polarisation of the magnet is such that the central portion between the points of contacts' with pole pieces i2 and I3. is of opposite polarity to that of the two ends, which themselves are polarised in the same direction as one another. The polarity of the various points will therefore be such that one end will be, say, of north polarity, the intermediate portion in contact with the adjacent pole piece of the main pole system will be of south polarity, the portion in contact with the other pole piece of the main pole system will again be of north polartiy and the other extreme end of the magnet will be of south olarity.

The pole pieces 15 and I6 are carried round the edge of the diaphragm and brought into proximity with the side of the diaphragm opposite to the side in proximity with pole pieces I2 and [3 but are not crossed over as in Figs. 5 and 7, since the polarisation of the magnet produces the same effect without this crossing over.

As compared with Figs. 5 and 7, the magnet must be longer for the arrangement of Fig. 11 since the magneto motive force VI is furnished by' that of the two outer sections, opposed by that of the middle portion.

On the other hand, the fluxes of the auxiliary gap system traverse the outer portions of the magnet only and the fluxes of the main gap system traverse the middle portion of the magnet only and no part of the magnet is traversed by both fluxes. In Figs. 5 and 7 however, both fluxes traverse, in the former, part, and in the latter, the whole of the magnet so that the magnet in these figures must be of greater cross section than in the case of Fig. 9.

.The' .electrical analogy illustrated in Fig. shows that the arrangement of Fig. 9 is the same in. principle as those of Figs. 5 and '7, in that cur- 10 may be applied to thearrangement of Fig. 11

by considering the elements E/Z as representing 7 the reluctance of the outer portions of the diaphragm in series with the gaps I! and I8 between pole pieces I5 and I6 respectively, and adjacent parts of the outer edge of the diaphragm. These'gaps Iliand 18 may beomitted and the diaphragm brought into contact with pole pieces [5 and I6, since in any event, the reluctance of such gaps is likely to be small compared with that of the outer portion of the diaphragm.

Here thetotal flux passes through the diaphragm material but the different components of the flux traverse different parts of the diaphragm, no single partof which is traversed by the whole flux. Saturation of the diaphragm between the parts adjacent to the gaps of the main gap system is thus avoided as with the other arrangements described;

Figs. 12, 13 and 14 are top plan, side cross sectionvand bottom plan respectively of a practical construction of an electro acoustic transducer of the type illustrated in Fig. 1, for use as a telephone earpiece insert.

The same numerical references are used in those figures asare used in Fig. 1, to denote corresponding elements. r

A cylindrical magnet l, polarised axially, has a base plate 3 seated on one end face and a top plate 6 seated on the other end face.

Fixed to the centre of base plate 3 is a cylindricalcentre pole piece 4 carrying a bobbin l9 having windings, (not shown), to carry speech currents. Fitting closely within the magnet is a cylindrical spacing member 20, of an axial length less than that of the magnet. Member 20 must be of non magnetic material and if made of electrically conducting material, inust not be a continuous ring since such a ring would act as a short circuited turn on the speech coil. One end of 20 rests on base plate 3, and on the other end face of 20 the diaphragm'rests, being held in position by the pull of the polarising flux from pole piece 4. i

The diaphragm edge is in proximity with the magnet at a part of the latter spaced away from the junction of the magnet and top plate 6.

Top plate 6 rests on the end face of the magnet remote from base plate 3, and between I and 8 is clamped a membrane 2| of impervious material, such as oiled silk, covering the diaphragm. The purpose of 2! is to keep moisture out of the interior of the receiver.

The central portion 22 of top plate 6 is dished inwards towards the diaphragm so as to present a disc-like pole surface thereto, substantially opposite to pole piece 4 on the other side of the diaphragm.

The sound apertures 23, four in number, are drilled into the sides of the depression in top plate 6. r

The sizeand number of these holes should be chosen so that the acoustic mass of the plug of air within-them in combination with the acoustic stiffness of the air space between plate 6 and the diaphragm forms an acoustically resonant system having a resonant frequency at the upper end of the frequency range which the transducer is required to handle. An acoustic arrangement of this type is described in British Patent No.

481,740. The flange of bobbin l9 nearest to the holes 25 communicating with a larger air space in the interior of the receiver, around the coils wound onrthe bobbin and bounded by spacing piece 20 and base plate 3. Spacing ring 20 is shown with an inward facingledge upon which rests the: said flange of bobbin l8 completingjthe wall'siof the saidiargerrairspaca. 1

i The .holes 25 are covered with porous material such as silk, to constitute an acoustic resistance between thesmall air space on one side of "the said bobbin flangeandjthe larger airspace on the other side of it.

The dimensions of these air spaces and the i holes 25 and the resistance material covering themare such thatthe acousticstifiness of the air space, the acoustic mass of theplugs of air in the holes and theacou'stic resistance ofthe material covering the holes: combine to term a damped acoustically anti-resonantsystein over a range of frequencies including the natural mechanical resonant frequency of the diaphragm.

.An acoustic system of this type is "described in British Patent No. 503,676;

The base plate 3 is pierced with holes 26 through drical covering 32 which projctsbeyondtopplate s and "disc 28, theprojec tin'g portions being pressed orspun over 6 andZB to hold; the components together axially.

Iclaim: H

1. An: electro-acoustic transducer comprising,

in combination: a diaphragm of magnetic material; a single permanent magnet memberj a main pole system including at leastone pole piece adjacent said diaphragm and a longitudinal portion of said diaphragm magnetically connected in series with at least a portion of said permanent magnet member; a main air gap system including a main air gap between each pole piece and the adjacent part of said diaphragm, said permanent magnet cooperating with said main pole system to produce a magnetic flux in said main air gap system and an auxiliary pole system including a magnetic path having at least one pole piece adjacent said diaphragm and magnetically connected in series with at least a portion of said permanent magnet member, said magnetic path being connected in series with a main air gap and in shunt with the longitudinal portion of the diaphragm adjacent said main air gap, said permanent magnet cooperating with said auxiliary pole system to produce an additional magnetic flux in said main air gap system to divert a portion of said first-mentioned magnetic flux passing through said adjacent part of the diaphragm.

2. An electro-acoustic transducer as defined in claim 1, wherein said main pole system comprises a first pole piece mounted adjacent one side of said diaphragm, and said auxiliary pole system comprises another pole piece mounted adjacent the other side of said diaphragm.

3. An electro-acoustic transducer as defined in claim 2, wherein said other pole piece consists of a plate having a central recess so as to present a substantially circular disc-like pole face to said diaphragm.

4. An electro-aooustic transducer as defined in claim 3, wherein said pole piece plate is provided with apertures for the ingress and egress of sound.

5. An electro-acoustic transducer as defined in adjacent the other side of said'diaphragmside 'of said diaphragm, and said auxiliary pole system comprises another. pole piece con'nected to the other pole of said magnetandarran ged 7. An electro-ac'oustic transducer asdfine'd'in claim 1, wherein said main pole systemcom'prises two first pole pieces each of which'is magnetically connected with an "opposite pole of'said permanent magnet and arranged adjacent said diaphragm, and said auxiliary pole system comprises two additional pole pieces each'of which is magnetically "connected with an opposite pole ofsaid permanent magne 'andarra nged adjacent said diaphragm.

8. An electro-acoustie transducer compi'ising, in combination: a diaphragm of'mag'netic material; a single permanent magnet member a main pole system including two polepieces positioned adjacent one side ofsaid-'diaphragmandieach magnetically connected with an opposite pole of said permanent magnet member; a main'air gap system including an air gap between each'pole piece and the adjacent part of said diaphragm, said permanent magnet cooperating with saidmain pole system to produce a magnetic flux in said main air gap system and an auxiliary pole system including two additional pole pieces positioned adjacent the other side of said diaphragm and each magnetically connected with an opposite pole of said permanent magnet member, said permanent magnet cooperating with said auxiliary pole system to produce an additional magnetic flux in said main air gap system to divert a portion of said first-mentioned magnetic flux passing through said adjacent part of the diaphragm.

9. An electro-acoustic transducer as defined in claim 1, wherein said main pole system comprises two first pole pieces, each of which is magnetically connected with an opposite pole of said permanent magnet and arranged adjacent one side of said diaphragm, and said auxiliary pole system comprises two further pole pieces each of which is connected with an. opposite pole of said permanent magnet and arranged adjacent the same side of the said diaphragm as the first pole pieces.

10. An electro-acoustic transducer comprising, in combination: a diaphragm of magnetic material; a single fiat permanent magnet member magnetized with consequent polarity along its length; a main pole system including two pole pieces adjacent said diaphragm and each magnetically connected with first opposite poles of said permanent magnet member; a main air gap system including an air gap between each pole piece and the adjacent part of said diaphragm, said permanent magnet cooperating with said main pole system to produce a magnetic flux in said main air gap system and an auxiliary pole system including two additional pole pieces adjacent said diaphragm and each magnetically :connected with additional opposite poles of said 11. An electro-acoustic transducer as defined in claim 1, wherein said permanent magnet is a fiat magnet polarized longitudinally, said main 'pole system comprises two first pole pieces each of which is magnetically connected to an opposite pole of said magnet and arranged adjacent said diaphragm, and said auxiliary pole system comprises two additional pole pieces each of which is magnetically connected to parts of the magnet different from the parts to which the first pole pieces are connected and arranged adjacent said diaphragm in such maner that each of said 'additional pole pieces is nearest, at the end thereof adjacent said diaphragm, to that one of said first pole pieces from which it is furthest away at their respective connecting points with the permanent magnet, and in which the polarity of each additional pole piece is'opposite to that of the-one first pole piece which is nearest to it adjacent the diaphragm.

12. An electro-acoustic transducer as defined in claim 1, wherein said main pole system car- 'ries windings adapted to pass audio frequency currents, further comprising fixed external contact members; two plates mounted as an assembly 'in the same plane and each plate having portions concentric with portions of the other plate so that contact can be made with the two plates by means of said external contact members in any position of rotation of said assembly in said plate about the axis of the aforesaid concentricity, each plate having one or more re-entrant portionsso that connections at points which are equidistant from said axis can be made on said reentrant portions of both plates, and means for connecting the ends of said windings to said two plates respectively.

13.'An electro-acoustic transducer as defined in claim 1, comprising a space which encloses a volume of air between'said diaphragm and said auxiliary pole system, and apertures in the pole piece of said system, the dimension and shape of said space and apertures being such that the acoustic stiffness of the space combined with the acoustic mass of the air in the apertures provides an acoustically resonant system with a resonant frequency substantially at the upper limit of frequencies with which the transducer is adapted to deal.

JAMES SAMUEL PATERSON ROBERTON.

REFERENCES CITED The following references are of record in the file of this patent: V

UNITED STATES PATENTS Number Name; a Date 679,405 Volkers July 30, 1901 1,660,864 Conrad Feb. 28, 1928 1,721,217 Hineline July lfi, 1929 1,830,160 French et al.- Nov. 3, 1931 2,069,817 Clarke Feb; 9, 1937 2,099,727 Duncan Nov. 23, 1937 2,249,158 Morrison July 15, 1941 2,360,796 Roberton Oct. 17, 1944 2,396,135 Terry et a1. Mar. 5, 1946 

